The invention relates to a method for adjusting a semiconductor disk relative to a radiation mask in x-ray photolithography as it is more precisely disclosed in the preamble of the main claim.
For the production of semiconductor components and integrated circuits in semiconductor technology photolithographic processes are carried out in which structures of a radiation mask are transmitted onto a radiation-sensitive lacquer layer on a semiconductor disk. During the total production processes it is necessary to bring coincidence to several layers of structures. For this reason the semiconductor disk must be very precisely adjusted in relation to the respective mask in the various exposure steps. Thereby the error in the adjusting of the masks relative to the semiconductor disks must be smaller than the smallest structure to be produced in the semiconductor disk. In the present production method for integrated circuits the structure transfer most often proceeds light-optically. The adjustment is carried out by simultaneous observation of two pairs of adjusting marks, whereby the one pair of adjusting marks is situated on the mask and the other pair is situated on the semiconductor disk. The observation is normally carried out light-optically with an adjusting microscope. However, for very precise adjustments in the range of one-half micron (0.5 .mu.m) the limits of the depth of focus of conventional light-optical microscopes is attained.
X-ray photolithography utilizes a lacquer sensitive to x-radiation and also utilizes x-radiation for reproducing the mask. Due to the short wavelength, diffraction phenomena are decreased in this type of reproduction so that the structure dimensions which can be reproduced are considerably smaller than in a reproduction with visible light. For this reason an adjustment with a precision of approximately one-tenth micron (0.1 .mu.m) has to be obtained in x-ray photolithography. As a so-called "proximity" copy is made in x-ray photolithography, in which a spacing, for example, of 50 .mu.m is kept between the semiconductor disk and the exposure mask, the depth of focus of a light-optical microscope is generally not sufficient to simultaneously view the adjusting mark on the semiconductor disk and the adjusting mark on the exposure mask in focus. Additional difficulties can occur in an adjusting system utilized for x-ray photolithography in that the material of the carrier on which the structures of the exposure mask, used as adjusting marks, are situated are not, or are only very poorly, permeable to light so that--due to this circumstance--a simultaneous sharp in-focus adjustment of the adjusting microscope with respect to the adjusting marks of the mask and the adjusting marks of the semiconductor disk is impaired.
An adjusting method for x-ray photolithography is known from "Solid State Technology" (1972), pages 21-26, in which the adjustment proceeds with the aid of x-rays. Thereto adjusting marks consisting of x-ray absorbing material are applied to the semiconductor disk and also to the radiation mask. After the x-rays have penetrated these adjusting marks, the intensity of radiation is measured with the aid of a detector, and depending upon the design of the two adjusting marks, the parts are relatively adjusted for an indication of radiation maximum or radiation minimum by the detector. Thereby, however, difficulties occur as the adjusting marks must be relatively thick in order to absorb the radiation sufficiently. The radiation penetrating the adjusting marks results in a high background illumination and decreases the adjusting precision. An additional difficulty results therefrom that the semiconductor disk per se absorbs a lot of radiation and thus decreases the signal intensity in the detector. In order to avoid these problems, "Solid State Technology" (1972), pages 21 through 26 suggests to thinly etch the semiconductor disk in the area of the adjusting marks and also to thinly etch the radiation mask in the area of the adjusting marks and the reproducing structures which absorb the x-rays, so that a sufficient x-ray intensity is available for the adjustment. However, for the thin-etching of the semiconductor disk additional method steps are necessary such as, for example, a boron-doping with a depth of approximately 5 .mu.m which takes care of the etch-stop and thus for an even thickness of the etched-off area during the thin-etching process. Aside from the fact that additional expense is necessary for the thin-etching, such semiconductor disks are sensitive to mechanical effects in the thin-etched areas such as, for example, jarrings occurring during the insertion into the exposure apparatus. The corresponding also holds true for the thin-etching of the exposure mask.